Sulfur oxides formed in the regenerator of a fluidized catalytic cracking unit during the combustion of the coke deposited on the catalyst particles may be controlled by including in the circulating inventory of the unit a sulfur oxide sorbent, such as reactive alumina, capable of sorbing the sulfur oxides in the regenerator and releasing them as hydrogen sulfide in the presence of hydrocarbons in the cracking vessel. See U.S. Pat. No. 4,071,436. The efficiency of such a sulfur oxides control system is enhanced by the presence of a sulfur dioxide oxidation promoter, such as platinum. These sulfur dioxide oxidation promoters aid the formation of sulfur trioxide in the regenerator which is more readily sorbed by the sulfur oxide sorbent than sulfur dioxide. However, a disadvantage of this method of controlling sulfur oxides is that the promoter has been observed to increase the amount of nitrogen oxides present in the regenerator flue gas. Since the oxides of nitrogen are themselves noxious gases, it is desirable to control the amount of nitrogen oxides, especially nitric oxide (NO), in the flue gas. The disclosure of U.S. Pat. No. 4,071,436 is herewith incorporated by reference into this specification.
U.S. Pat. No. 3,900,554 teaches a method for selectively reducing nitric oxide by mixing an effluent stream containing nitric oxide with ammonia and oxygen. The resulting mixture is subjected to a high temperature, preferably in the presence of a reducing material such as hydrogen. Various other processes have been described for reducing nitrogen oxides using ammonia, but most of these methods utilize a catalyst.